In many wireless communications systems, the power amplifier (PA) in the transmitter is required to be very linear, in addition to being able to simultaneously amplify many radio channels (frequencies) spread across a fairly wide bandwidth. It also has to do this efficiently, in order to reduce power consumption and need for cooling, and to increase its longevity. High linearity is required, since nonlinear amplifiers would cause leakage of interfering signal energy between channels.
The amplitude probability density of a mix of sufficiently many independent radio frequency (RF) channels, or of a multi-user CDMA (Code Division Multiple Access) signal, tends to be close to a Rayleigh distribution having a large peak-to-average power ratio. Since a conventional RF power amplifier generally has an efficiency proportional to its output amplitude, its average efficiency is very low for such signals.
In response to the low efficiency of conventional linear power amplifiers, many methods have been proposed. Two of the most promising are the Chireix outphasing method [1], and the Doherty method [2].
To minimize the costs for producing the efficient power amplifiers described in the previous sections, one would like to avoid trimming. Since component values and electrical lengths of transmission lines vary between the produced amplifiers, they will all be more or less detuned or off balance. One problem that so far has remained unsolved is how to obtain maximum efficiency (i.e. best possible under such conditions) from an imperfect composite amplifier.